Earlier quoted context omitted.
The idea that you can't be great at very different things isn't confined to software engineering. I think it's because most people can only master one thing so they can't imagine other people working differently
To be perfectly fair though even “mastering” backend is quite close to impossible. You’ve probably heard of the 10,000 hour rule, it’s a hard and fast rule to say that it takes roughly 10,000 hours of doing something to fully master it. Maybe not true in all cases; but consider the suite of tools you have to be practicing every day to do even a single role: VCS/DVCS, shell, OS fundamentals, build-systems, networking,…
Nevertheless, I have a solid understanding on all the things you mentioned and if you need help give me a holler because my rates are fair and I'm open to opportunities